It is a conventional practice to monitor the speed of an electric motor in order to effect motor control actions or to prevent damage to the motor when certain stall conditions occur. For example, a motor may be protected against stall conditions by a system which senses the motor speed and, when it falls to an unacceptably low level and overheats, acts to cut-off the power supply to the motor or otherwise generate an appropriate motor control function. Thus, U.S. Pat. No. 4,504,881 to Wada discloses a motor protection system which comprises a transducer that generates a signal having a frequency proportional to motor speed. The motor speed signal is applied to a control circuit in a feedback terminal and also to the reset terminal of a counter driven by a clock signal. If the motor speed decreases to an undesirably low speed where there is a danger of overheating, the counter times out and applies a signal through a flipflop circuit to turn a transistor in the motor supply circuit off and cuts off power to the motor.
U.S. Pat. No. 4,245,370 to Baker discloses a protective circuit for a vacuum cleaner motor in which a frequency proportional signal generated by a Hall effect transducer is applied to two timers having time-out intervals associated with upper end lower threshold speeds. When the motor reaches the upper threshold speed, the interval between the period of the motor speed signal is sufficiently great that one timer times out to generate a signal energizing a caution light. If the motor speed falls to the lower threshold value resulting in a greater period for the frequency proportional signal, the second timer times out and generates a function to open a switch in the motor supply circuit.
Motor protection systems are also used in the oil industry for the protection of electric motors employed as prime movers in sucker-rod type pumping units. Sucker-rod pumping units are widely used in the oil industry in order to recover fluids from wells extending into subterranean formations. Such units include a sucker-rod string which extends into the well to drive a downhole pump and means at the surface of the well for reciprocating the rod string. Typical of such units are the so-call "beam type" pumping units in which the sucker-rod string is suspended from a walking beam which is pivotally mounted on a Sampson post and driven by an electric motor. The load on the electric motor varies widely during each pumping cycle and it is a conventional practice to monitor the operation of the unit and to shutdown the motor upon the occurrence of an unacceptable fault condition. For example, U.S. Pat. No. 3,778,694 to Hubby discloses a system for detecting a "pump off" condition by monitoring the load on the motor during the downstroke of the pumping unit. When the system detects a motor load which is abnormally low in comparison with a predetermined standard, it acts to remove power from the motor.